1. Field of the Invention
The present invention pertains to a method of plasma etching recessed structures such as a deep trench in a substrate. The plasma source gas used to generate plasma etchant species is modulated during the etching process. The method is particularly useful in combination with particular plasma source gases for etching trenches in a silicon substrate.
2. Brief Description of the Background Art
Deep recessed structure etching is one of the principal technologies currently being used to fabricate microstructure devices, and is an enabling technology for many microelectromechanical systems (MEMS) applications. Strict control of the etch profile is required for these new, complex devices to perform satisfactorily. In a number of instances, it is desired to etch a vertical side wall, where vertical typically means that a taper angle formed by the side wall with a horizontal line drawn at the base of the side wall ranges from 85° up to 92°. Obtaining a controlled sidewall taper angle of 85° to 92° in combination with a smooth sidewall surface has proved a difficult task in many instances. In addition, microloading has been observed during the etching of substrates where some recess features are densely positioned while other recess features are isolated.
Trenches with a sidewall positive taper angle of 85° up to 92° are useful in a variety of MEMS devices such as optical switches, tuneable capacitors, accelerometers, and gyroscopes, by way of example and not by way of limitation.
Plasma etching of deeply recessed structures, where the depth of the recess is at least 10 μm, typically require a combination of reactive chemical etching with physical etching, which typically employs ion bombardment. The physical etching enables anisotropic, directional etching necessary to produce vertical sidewalls on an etched trench, for example. However, to obtain a vertical sidewall, it is necessary to control the incoming ions so that they strike the bottom of the feature being etched, but not the already etched surfaces extending above the bottom of the feature.
Numerous processing techniques have been proposed to solve the challenges related to providing control over the shape (sidewall taper, for example) of the etched profile, while simultaneously providing a smooth surface on the etched sidewall. One technique for forming trenches having nearly vertical sidewalls employs a protective coating in the area of the opening to the trench. The material used to form the coating is resistant to the etchant being used to etch the trench. The coating may be applied continuously or may be applied at specific points in the trench formation process. For a more detailed description of this method, one skilled in the art should read U.S. Pat. No. 4,533,430 to Robert W. Bower, issued Aug. 6, 1985. In a related method, a silicon substrate is covered with a patterned mask which exposes select areas of a silicon substrate to plasma etching. Anisotropic etching is accomplished using alternating plasma etching and polymer formation steps. Additional information about this method may be found in Japanese Patent No. JP 63-13334, issued in 1988.
Some etching methods recommend the use of the same gas mixture during plasma etching of a feature and during formation of a protective film to protect etched surfaces. In one method, by changing the substrate bias, the process is said to be switched between a first state in which the primary reaction is substrate etching and a second state in which the primary reaction is deposition of a film on the substrate surface. For a detailed description of this technique, please refer to U.S. Pat. No. 4,795,529 to Kawasaki et al., issued Jan. 3, 1989.
Another description of the use of alternating plasma etching and polymer formation steps is found in U.S. Pat. No. 5, 501,893 to Laermer et al., issued Mar. 26, 1996. The etch and polymerization steps are carried out in an alternating, repetitive manner until etching is complete. Subsequently, in U.S. Pat. No. 6,284,148 B1, issued Sep. 4, 2001, Laermer et al. describe a method in which the quantity of polymer deposited decreases in the course of the polymer deposition steps.
In a related patent, a method is described for etching a trench in a semiconductor substrate using alternatively reactive ion etching and deposition of a passivation layer by chemical vapor deposition. The method includes varying one or more of a number of process variables with time during the etch process. The variation in process parameters is generally illustrated as being periodic, where the periodic variation corresponds to at least one sinusoidal, square, or sawtooth waveform. In one preferred embodiment, the process parameter varied over time is subjected to a ramped variation. Pumping out of the process chamber between either the steps within a given cycle or between cycles is also described. For more details of the process parameter variations described above, one skilled in the art may read U.S. Pat. No. 6,051,503 to Bhardwaj et al., issued Apr. 18, 2000.
The teachings of Bhardwaj et al. add another layer of complexity to the already complex processes described by Laermer et al. for the etching of deep trenches in silicon substrates. However, this increase in process complexity is said to address or reduce various problems in the etch process described in the Laermer et al. patents.
After reading the patents referred to above and a number of additional patents pertaining to the etching of deep trenches in silicon, it becomes readily apparent there is a need for a simplified, streamlined etch process which provides vertical recess feature sidewalls which are smooth (having a surface roughness of about 1 μm or less.) Further, there is a need for a process which reduces the amount of microloading which occurs when dense and isolated features are etched on the same substrate during the same etch process.